New Tub?Shower Installation

I'm a DIYer finishing my basement. The floor is concrete, which I will tile. The builder left a rough-in for a tub and toilet. I've framed the area for the tub and plan to install a one piece acrylic tub/shower combo, Delta model 226032A. I get how to do all the plumbing for the supply/mixer/showerhead. I understand how to connect the drain assembly to the tub. I'm lost on how to attach the assembly to the rough-in stub that the builder left. I assume that I put in a trap, with the opening lined up with the straight pipe coming off the drain assembly, so that the drain slides into the trap. But how am I supposed to attach those pipes? The trap is down inside the concrete, and it will all be covered by the tub once I put the thing together. I'm confused.

A plumber would measure carefully, apply cement, set the tub in place and be done. Practice helps immensely and once you add the cement, you only get one chance. If you don't have an access panel on that wet end, that's about the only way to do it.

A plumber would prefit the drain assembly to the tub then remove it. He would measure carefully and attach the drain to the drain pipe, set the tub in place OVER it, then connect the drain assembly to the tub.

Thank you both for your replies. They are very different approaches. I think the one that makes the most sense is to set the drain assembly and then install the tub onto it. I'd like to see if anyone else has any input. Also, for the drain stub that the builder left in the rough-in box: it's in gravel inside a ~8"x10" cut out in the concrete. I haven't dug into it yet (the house is several hours away & I only get there a couple of weekends a month). There is a PVC pipe running horizontally coming from the wall with about 4" showing out of the gravel. Is it typical for this to have a trap already in the run, or do I need to add one? What about venting, is the drain usually on the main vent stack?

Most plumbers box out and leave the trap installation for the installing plumber after the walls have been framed in.
If a trap had been installed at rough-in, which it seems this one wasn't, it would almost never be spotted correctly for the tub anyway. Too much can happen between the concrete pour and the framing of the walls.

HJ has it right; Install the tub waste and overflow to the tub first, remove it from the tub and install the drain to the p-trap in the floor. Then you can put the tub in and attach the fittings through the tub after it's in place. It's much easier to get the tub drain right, with the tub in the open and accessible.

Thank you HJ and Terry. That makes sense. But I have a feeling that about halfway through this I'll have a new appreciation for you guys that do this for a living, and an expanded four letter word vocabulary.